The present invention generally relates to semiconductor devices and a fabrication process thereof. More particularly, the present invention relates to a method of forming a dielectric film and fabrication process of a non-volatile semiconductor memory device capable of rewriting information electrically, including a flash memory device.
There are various volatile memory devices such as DRAMs and SRAMs. Further, there are nonvolatile memory devices such as a mask ROM, PROM, EPROM, EEPROM, and the like. Particularly, a flash memory device is an EEPROM having a single transistor for one memory cell and has an advantageous feature of small cell size, large storage capacity and low power consumption. Thus, intensive efforts are being made on the improvement of flash memory devices. In order that a flash memory device can be used stably over a long interval of time with low voltage, it is essential to use a uniform insulation film having high film quality.
First, the construction of a conventional flash memory device will be explained with reference to FIG. 1 showing the concept of a generally used flash memory device having a so-called stacked gate structure.
Referring to FIG. 1, the flash memory device is constructed on a silicon substrate 1700 and includes a source region 1701 and a drain region 1702 formed in the silicon substrate 1700, a tunneling gate oxide film 1703 formed on the silicon substrate 1700 between the source region 1701 and the drain region 1702, and a floating gate 1704 formed on the tunneling gate oxide film 1703, wherein there is formed a consecutive stacking of a silicon oxide film 1705, a silicon nitride film 1706 and a silicon oxide film 1707 on the floating gate 1704, and a control gate 1708 is formed further on the silicon oxide film 1707. Thus, the flash memory of such a stacked structure includes a stacked structure in which the floating gate 1704 and the control gate 1708 sandwich an insulating structure formed of the insulation films 1705, 1706 and 1707 therebetween.
The insulating structure provided between the floating gate 1704 and the control gate 1705 is generally formed to have a so-called ONO structure in which the nitride film 1706 is sandwiched by the oxide films 1705 and 1707 for suppressing the leakage current between the floating gate 1704 and the control gate 1705. In an ordinary flash memory device, the tunneling gate oxide film 1703 and the silicon oxide film 1705 are formed by a thermal oxidation process, while the silicon nitride film 1706 and the silicon oxide film 1707 are formed by a CVD process. The silicon oxide film 1705 may be formed by a CVD process. The tunneling gate oxide film 1703 has a thickness of about 8 nm, while the insulation films 1705, 1706 and 1707 are formed to have a total thickness of about 15 nm in terms of oxide equivalent thickness. Further, a low-voltage transistor having a gate oxide film of 3-7 nm in thickness and a high-voltage transistor having a gate oxide film of 15-30 nm in thickness are formed on the same silicon in addition to the foregoing memory cell.
In the flash memory cell having such a stacked structure, a voltage of about 5-7V is applied for example to the drain 1702 when writing information together with a high voltage larger than 12V applied to the control gate 1708. By doing so, the channel hot electrons formed in the vicinity of the drain region 1702 are accumulated in the floating gate via the tunneling insulation film 1703. When erasing the electrons thus accumulated, the drain region 1702 is made floating and the control gate 1708 is grounded. Further, a high voltage larger than 12V is applied to the source region 1701 for pulling out the electrons accumulated in the floating gate 1704 to the source region 1701.
Such a conventional flash memory device, on the other hand, requires a high voltage at the time of writing or erasing of information, while the use of such a high voltage tends to cause a large substrate current. The large substrate current, in turn, causes the problem of deterioration of the tunneling insulation film and hence the degradation of device performance. Further, the use of such a high voltage limits the number of times rewriting of information can be made in a flash memory device and also causes the problem of erroneous erasing.
The reason a high voltage has been needed in conventional flash memory devices is that the ONO film, formed of the insulation films 1705, 1706 and 1707, has a large thickness.
In the conventional art of film formation, there has been a problem, when a high-temperature process such as thermal oxidation process is used in the process of forming an oxide film such as the insulation film 1705 on the floating gate 1704, in that the quality of the interface between the polysilicon gate 1704 and the oxide film tends to become poor due to the thermal budget effect, etc. In order to avoid this problem, one may use a low temperature process such as CVD process for forming the oxide film. However, it has been difficult to form a high-quality oxide film according to such a low-temperature process. Because of this reason, conventional flash memory devices had to use a large thickness for the insulation films 1705, 1706 and 1707 so as to suppress the leakage current.
However, the use of large thickness for the insulation films 1705, 1706 and 1707 in these conventional flash memory devices has caused the problem in that it is necessary to use a large writing voltage and also a large erasing voltage. As a result of using large writing voltage and large erasing voltage, it has been necessary to form the tunneling gate insulation film 1703 with large thickness so as to endure the large voltage used.
Accordingly, it is a general object of the present invention to provide a novel and useful flash memory device and fabrication process thereof and further a method of forming an insulation film, wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a high-performance flash memory device having a high-quality insulation film that is formed at a low temperature process, the thickness of the tunneling gate insulation film or the thickness of the insulation film between the floating gate and the control gate can be reduced successfully without causing the problem of leakage current, and enabling writing and erasing at low voltage.
Another object of the present invention is to provide a method of forming an insulation film wherein a high-quality insulation film can be formed on polysilicon.
Another object of the present invention is to provide a flash memory device, comprising:
a silicon substrate,
a first electrode formed on the silicon substrate with a tunneling insulation film interposed therebetween, and
a second electrode formed on the first electrode with an insulation film interposed therebetween,
said insulation film having a stacked structure including at least one silicon oxide film and one silicon nitride film, at least a part of said silicon oxide film containing Kr with a surface density of 1010 cmxe2x88x922 or more.
According to the present invention, the quality of the insulation film used in a flash memory device between a floating gate electrode and a control gate electrode is improved by forming the insulation film by an oxidation reaction or nitriding reaction conducted in Ar or Kr plasma in which atomic state oxygen O* or hydrogen nitride radicals NH* are formed efficiently. Further, it becomes possible to reduce the thickness of the insulation film without causing unwanted increase of leakage current. As a result, the flash memory device of the present invention can operate at high speed with low voltage and has a long lifetime.
Another object of the present invention is to provide a method of fabricating a flash memory device comprising a silicon substrate, a first electrode of polysilicon formed on the silicon substrate with an insulation film interposed therebetween, and a second electrode formed on the first electrode with an inter-electrode insulation film interposed therebetween, said inter-electrode insulation film having a stacked structure containing at least one silicon oxide film and one silicon nitride film,
said silicon oxide film being formed by the step of exposing a silicon oxide film deposited by a CVD process to atomic state oxygen O* formed by microwave excitation of plasma in a mixed gas of an oxygen-containing gas and an inert gas predominantly of a Kr gas.
According to the present invention, an oxide film having excellent leakage current characteristic is obtained for the inter-electrode insulation film, and it becomes possible to form a flash memory having a simple structure, capable of holding electric charges in the floating gate electrode stably, and is operable at a low driving voltage.
Another object of the present invention is to provide a fabrication process of a flash memory device comprising a silicon substrate, a first electrode of polysilicon formed on the silicon substrate with an insulation film interposed therebetween, and a second electrode formed on the first electrode with an inter-electrode insulation film interposed therebetween, said inter-electrode insulation film having a stacked structure including at least one silicon oxide film and one silicon nitride film,
said silicon nitride film being formed by exposing a silicon nitride film deposited by a CVD process to hydrogen nitride radicals NH* formed by microwave excitation of plasma in a mixed gas of an NH3 gas or alternatively a gas containing N2 and H2 and a gas predominantly formed of an Ar or Kr gas.
According to the present invention, a nitride film having excellent leakage current characteristic suitable for the inter-electrode insulation film is obtained. Thus, it becomes possible to realize a flash memory having a simple construction and is capable of holding electric charges stably in the floating gate electrode. The flash memory thus obtained is operable at a low driving voltage.
Another object of the present invention is to provide a method of forming a silicon oxide film, comprising the steps of:
depositing a polysilicon film on a substrate; and
forming a silicon oxide film on a surface of said polysilicon film by exposing the surface of said polysilicon film to atomic state oxygen O* formed by microwave excitation of plasma in a mixed gas of a gas containing oxygen and an inert gas predominantly of a Kr gas.
According to the present invention, it becomes possible to form a homogeneous silicon oxide film on a polysilicon film with uniform thickness irrespective of the orientation of the silicon crystals therein. The silicon oxide film thus formed has excellent leakage current characteristic comparative to that of a thermal oxide film and causes a Fowler-Nordheim tunneling similarly to the case of a thermal oxide film.
Another object of the present invention is to provide a method of forming a silicon nitride film, comprising the steps of:
depositing a polysilicon film on a substrate; and
forming a nitride film on a surface of said polysilicon film by exposing the surface of said polysilicon film to hydrogen nitride radicals NH* formed by microwave excitation of plasma in a mixed gas of a gas containing nitrogen and hydrogen as constituent elements and an inert gas predominantly of an Ar gas or a Kr gas.
According to the present invention, it becomes possible to form a nitride film of excellent characteristic on the surface of a polysilicon film.
Another object of the present invention is to provide a method of forming a dielectric film, comprising the steps of:
depositing a polysilicon film on a substrate; and
converting a surface of said polysilicon film into a dielectric film by exposing said polysilicon film to a microwave-excited plasma formed in a mixed gas of an inert gas predominantly of Ar or Kr and a gas containing oxygen as a constituent element and a gas containing nitrogen as a constituent element.
According to the present invention, it becomes possible to form an oxynitride film having excellent characteristic on the surface of a polysilicon film.
Another object of the present invention is to provide a method of fabricating a flash memory having a silicon substrate, a first electrode of polysilicon formed on said silicon substrate with an insulation film interposed therebetween, and a second electrode formed on said first electrode with an inter-electrode oxide film interposed therebetween, said inter-electrode oxide film being formed by the steps of:
depositing a polysilicon film on said silicon substrate as said first electrode; and
exposing a surface of said polysilicon film to atomic state oxygen O* formed by microwave excitation of plasma in a mixed gas of a gas containing oxygen and an inert gas predominantly of a Kr gas.
According to the present invention, an oxide film having excellent leakage current characteristic is obtained for the inter-electrode insulation film, and it becomes possible to realize a flash memory having a simple construction and is capable of holding electric charges in the floating gate electrode stably. The flash memory thus formed is operable at a low driving voltage.
Another object of the present invention is to provide a method of fabricating a flash memory having a silicon substrate, a first electrode of polysilicon formed on said silicon substrate with an oxide film interposed therebetween, and a second electrode of polysilicon formed on said first electrode with an inter-electrode nitride film interposed therebetween, said inter-electrode nitride film being formed by the steps of:
depositing a polysilicon film on said silicon substrate as said first electrode; and
exposing a surface of said polysilicon film to hydrogen nitride radicals NH* formed by microwave excitation of plasma in a mixed gas of a gas containing nitrogen and hydrogen and an inert gas predominantly of an Ar gas or a Kr gas.
According to the present invention, a nitride film having excellent leakage current characteristic is obtained for the inter-electrode nitride film and it becomes possible to realize a flash memory having a simple construction and is capable of holding electric charges in the floating gate electrode stably. The flash memory thus formed is operable at a low driving voltage.
Another object of the present invention is to provide a method of fabricating a flash memory having a silicon substrate, a first electrode of polysilicon formed on said silicon substrate with insulation film interposed therebetween, and a second electrode of polysilicon formed on said first electrode with an inter-electrode oxynitride film interposed therebetween, said inter-electrode oxynitride film being formed by the steps of:
depositing a polysilicon film on said silicon substrate as said first electrode; and
converting a surface of said polysilicon film into a silicon oxynitride film by exposing said polysilicon film to microwave excited plasma formed in a mixed gas of an inert gas predominantly of Ar or Kr and a gas containing oxygen and nitrogen.
According to the present invention, an oxynitride film having excellent leakage current characteristic is obtained for the inter-electrode insulation film, and it becomes possible to realize a flash memory capable of holding electric charges stably in the floating gate electrode. The flash memory thus formed is operable at a low driving voltage.
Another object of the present invention is to provide a method of forming a silicon oxide film on a polysilicon film, comprising the steps of:
forming atomic state oxygen O* in a processing vessel of a microwave processing apparatus, said microwave processing apparatus including: a shower plate in a part of said processing vessel such that said shower plate extends parallel to a substrate to be processed, said shower plate having a plurality of apertures for supplying a plasma gas toward said substrate; and a microwave radiation antenna emitting a microwave into said processing vessel via said shower plate, by supplying a gas predominantly of Kr and a gas containing oxygen into said processing vessel via said shower plate and further by supplying said microwave into said processing vessel from said microwave radiation antenna through said shower plate; and
forming a silicon oxide film by causing oxidation in a surface of a polysilicon film formed on said substrate by said plasma in said processing vessel.
According to the present invention, atomic state oxygen that cause oxidation in a polysilicon film are formed efficiently by inducing high-density plasma of low electron temperature in the processing chamber as a result of microwave excitation of the plasma gas supplied uniformly from the shower plate. The silicon oxide film thus formed by the Kr plasma is irrelevant to the crystal orientation of the Si crystals on which the silicon oxide film is formed. Thus, the silicon oxide film is formed uniformly on the polysilicon film. The silicon oxide film contains little surface states and is characterized by small leakage current. According to the present invention, the oxidation processing of the polysilicon film can be conducted at a low temperature of 550xc2x0 C. or less, and there occurs no substantial grain growth in the polysilicon film even when such an oxidation process is conducted. Thus, the problem of concentration of electric field, and the like, which arises with such a grain growth is avoided.
Another object of the present invention is to provide a method of forming a silicon nitride film on a polysilicon film, said method comprising the steps of:
forming plasma containing hydrogen nitride radicals NH* in a processing vessel of a microwave processing apparatus, said microwave processing apparatus including: a shower plate in a part of said processing vessel so as to extend parallel to a substrate to be processed, said shower plate having a plurality of apertures for supplying a plasma gas to said substrate; and a microwave radiation antenna emitting a microwave into said processing vessel via said shower plate, by supplying a gas predominantly of Ar or Kr and a gas containing nitrogen and hydrogen into said processing vessel from said shower plate and by further supplying said microwave into said processing vessel from said microwave radiation antenna through said shower plate; and
forming a silicon nitride film by nitriding a surface of a polysilicon film formed on said substrate by said plasma in said processing vessel.
According to the present invention, hydrogen nitride radicals NH* that cause nitridation in the polysilicon film are formed efficiently by inducing high-density plasma having a low electron temperature in the processing chamber by microwave excitation of the plasma gas supplied uniformly from the shower plate. The silicon nitride film thus formed by the Kr plasma has an advantageous feature of small leakage current in spite of the fact that the silicon nitride film is formed at a low temperature.
Another object of the present invention is to provide a method of fabricating a flash memory device, said flash memory device having a silicon substrate and including a first electrode formed on said silicon substrate with a tunneling insulation film interposed therebetween and a second electrode formed on said first electrode with an insulation film interposed therebetween, said insulation film having a stacked structure containing at least one silicon oxide film and one silicon nitride film, said silicon oxide film being formed by the steps of:
introducing a gas containing oxygen and a gas predominantly of a Kr gas into a processing chamber, and causing microwave excitation of plasma in said processing chamber.
According to the present invention, it becomes possible to oxidize the surface of the first electrode at low temperature, by conducting the oxidation processing in the Kr plasma in which atomic state oxygen O* are formed efficiently. As a result, an oxide film containing small surface states and is characterized by small leakage current can be obtained for the desired silicon oxide film.
Another object of the present invention is to provide a fabrication process of a flash memory device having a silicon substrate, a first electrode formed on said silicon substrate with a tunneling insulation film interposed therebetween, and a second electrode formed on said first electrode with an insulation film interposed therebetween, said insulation film having a stacked structure containing at least one silicon oxide film and one silicon nitride film,
said silicon nitride film being formed by introducing an NH3 gas or a gas containing N2 and H2 and a gas predominantly of Ar or Kr into a processing chamber, and causing microwave excitation of plasma in said processing chamber.
According to the present invention, it becomes possible to nitride the surface of the first electrode at low temperature by conducting the nitridation in the plasma of Ar or Kr in which the hydrogen nitride radicals NH* are formed efficiently.
Other objects and further features of the present invention will become apparent from the following detailed description of the invention when read in conjunction with the attached drawings.